One class of opto-electrical devices is that using an organic material for light emission (or detection in the case of photovoltaic cells and the like). The basic structure of these devices is a light emissive organic layer, for instance a film of a poly (p-phenylenevinylene) (“PPV”) or polyfluorene, sandwiched between a cathode for injecting negative charge carriers (electrons) and an anode for injecting positive charge carriers (holes) into the organic layer. The electrons and holes combine in the organic layer generating photons. In WO90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline) aluminium (“Alq3”). In a practical device one of the electrodes is transparent, to allow the photons to escape the device.
A typical organic light-emissive device (“OLED”) is fabricated on a glass or plastic substrate coated with a transparent anode such as indium-tin-oxide (“ITO”). A layer of a thin film of at least one electroluminescent organic material covers the first electrode. Finally, a cathode covers the layer of electroluminescent organic material. The cathode is typically a metal or alloy and may comprise a single layer, such as aluminium, or a plurality of layers such as calcium and aluminium.
In operation, holes are injected into the device through the anode and electrons are injected into the device through the cathode. The holes and electrons combine in the organic electroluminescent layer to form an exciton which then undergoes radiative decay to give light (in light detecting devices this process essentially runs in reverse).
These devices have great potential for displays. However, there are several significant problems. One is to make the device efficient, particularly as measured by its external power efficiency and its external quantum efficiency. Another is to optimise (e.g. to reduce) the voltage at which peak efficiency is obtained. Another is to stabilise the voltage characteristics of the device over time. Another is to increase the lifetime of the device.
To this end, numerous modifications have been made to the basic device structure described above in order to solve one or more of these problems.
One such modification is the provision of a layer of conductive polymer between the light-emissive organic layer and one of the electrodes. It has been found that the provision of such a conductive polymer layer can improve the turn-on voltage, the brightness of the device at low voltage, the efficiency, the lifetime and the stability of the device. In order to achieve these benefits these conductive polymer layers typically may have a sheet resistance less than 106 Ohms/square, the conductivity being controllable by doping of the polymer layer. It may be advantageous in some device arrangements to not have too high a conductivity. For example, if a plurality of electrodes are provided in a device but only one continuous layer of conductive polymer extending over all the electrodes, then too high a conductivity can lead to lateral conduction and shorting between electrodes.
The conductive polymer layer may also be selected to have a suitable workfunction so as to aid in hole or electron injection and/or to block holes or electrons. There are thus two key electrical features: the overall conductivity of the conductive polymer composition; and the workfunction of the conductive polymer composition. The stability of the composition and reactivity with other components in a device will also be critical in providing an acceptable lifetime for a practical device. The processability of the composition will be critical for ease of manufacture.
Conductive polymer formulations are discussed in the applicant's earlier application GB-A-0428444.4. There is an ongoing need to optimise the organic formulations used in these devices both in the light emitting layer and the conductive polymer layer.
OLEDs can provide a particularly advantageous form of electro-optic display. They are bright, colourful, fast-switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. Organic (which here includes organometallic) LEDs may be fabricated using either polymers or small molecules in a range of colours (or in multi-coloured displays), depending upon the materials used. As previously described, a typical OLED device comprises two layers of organic material, one of which is a layer of light emitting material such as a light emitting polymer (LEP), oligomer or a light emitting low molecular weight material, and the other of which is a conductive polymer layer, for example a layer of a hole transporting material such as a polythiophene derivative or a polyaniline derivative.
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-colour pixellated display. A multicoloured display may be constructed using groups of red, green, and blue emitting pixels. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel whilst passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image.
FIG. 1 shows a vertical cross section through an example of an OLED device 100. In an active matrix display, part of the area of a pixel is occupied by associated drive circuitry (not shown in FIG. 1). The structure of the device is somewhat simplified for the purposes of illustration.
The OLED 100 comprises a substrate 102, typically 0.7 mm or 1.1 mm glass but optionally clear plastic, on which an anode layer 106 has been deposited. The anode layer typically comprises around 150 nm thickness of ITO (indium tin oxide), over which is provided a metal contact layer, typically around 500 nm of aluminium, sometimes referred to as anode metal. Glass substrates coated with ITO and contact metal may be purchased from Corning, USA. The contact metal (and optionally the ITO) is patterned as desired so that it does not obscure the display, by a conventional process of photolithography followed by etching.
A substantially transparent hole transport layer 108a is provided over the anode metal, followed by an electroluminescent layer 108b. Banks 112 may be formed on the substrate, for example from positive or negative photoresist material, to define wells 114 into which these active organic layers may be selectively deposited, for example by a droplet deposition or inkjet printing technique. The wells thus define light emitting areas or pixels of the display.
A cathode layer 110 is then applied by, say, physical vapour deposition. The cathode layer typically comprises a low work function metal such as calcium or barium covered with a thicker, capping layer of aluminium and optionally including an additional layer immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching. Mutual electrical isolation of cathode lines may achieved through the use of cathode separators (element 302 of FIG. 3b). Typically a number of displays are fabricated on a single substrate and at the end of the fabrication process the substrate is scribed, and the displays separated. An encapsulant such as a glass sheet or a metal can is utilized to inhibit oxidation and moisture ingress.
Organic LEDs of this general type may be fabricated using a range of materials including polymers, dendrimers, and so-called small molecules, to emit over a range of wavelengths at varying drive voltages and efficiencies. Examples of polymer-based OLED materials are described in WO90/13148, WO95/06400 and WO99/48160; examples of dendrimer-based materials are described in WO 99/21935 and WO 02/067343; and examples of small molecule OLED materials are described in U.S. Pat. No. 4,539,507. The aforementioned polymers, dendrimers and small molecules emit light by radiative decay of singlet excitons (fluorescence). However, up to 75% of excitons are triplet excitons which normally undergo non-radiative decay. Electroluminescence by radiative decay of triplet excitons (phosphorescence) is disclosed in, for example, “Very high-efficiency green organic light-emitting devices based on electrophosphorescence” M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, and S. R. Forrest Applied Physics Letters, Vol. 75(1) pp. 4-6, Jul. 5, 1999”. In the case of a polymer-based OLED, layers 108 comprise a hole transport layer 108a and a light emitting polymer (LEP) electroluminescent layer 108b. The electroluminescent layer may comprise, for example, around 70 nm (dry) thickness of PPV (poly(p-phenylenevinylene)) and the hole transport layer, which helps match the hole energy levels of the anode layer and of the electroluminescent layer, may comprise, for example, around 50-200 nm, preferably around 150 nm (dry) thickness of PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene).
FIG. 2 shows a view from above (that is, not through the substrate) of a portion of a three-colour active matrix pixellated OLED display 200 after deposition of one of the active colour layers. The figure shows an array of banks 112 and wells 114 defining pixels of the display.
FIG. 3a shows a view from above of a substrate 300 for inkjet printing a passive matrix OLED display. FIG. 3b shows a cross-section through the substrate of FIG. 3a along line Y-Y′.
Referring to FIGS. 3a and 3b, the substrate is provided with a plurality of cathode undercut separators 302 to separate adjacent cathode lines (which will be deposited in regions 304). A plurality of wells 308 is defined by banks 310, constructed around the perimeter of each well 308 and leaving an anode layer 306 exposed at the base of the well. The edges or faces of the banks are tapered onto the surface of the substrate as shown, heretofore at an angle of between 10 and 40 degrees. The banks present a hydrophobic surface in order that they are not wetted by the solution of deposited organic material and thus assist in containing the deposited material within a well. This is achieved by treatment of a bank material such as polyimide with an O2/CF4 plasma as disclosed in EP 0989778. Alternatively, the plasma treatment step may be avoided by use of a fluorinated material such as a fluorinated polyimide as disclosed in WO 03/083960.
As previously mentioned, the bank and separator structures may be formed from resist material, for example using a positive (or negative) resist for the banks and a negative (or positive) resist for the separators; both these resists may be based upon polyimide and spin coated onto the substrate, or a fluorinated or fluorinated-like photoresist may be employed. In the example shown the cathode separators are around 5 μm in height and approximately 20 μm wide. Banks are generally between 20 μm and 100 μm in width and in the example shown have a 4 μm taper at each edge (so that the banks are around 1 μm in height). The pixels of FIG. 3a are approximately 300 μm square but, as described later, the size of a pixel can vary considerably, depending upon the intended application.
The deposition of material for organic light emitting diodes (OLEDs) using ink jet printing techniques are described in a number of documents including, for example: T. R. Hebner, C. C. Wu, D. Marcy, M. H. Lu and J. C. Sturm, “Ink-jet Printing of doped Polymers for Organic Light Emitting Devices”, Applied Physics Letters, Vol. 72, No. 5, pp. 519-521, 1998; Y. Yang, “Review of Recent Progress on Polymer Electroluminescent Devices,” SPIE Photonics West: Optoelectronics '98, Conf. 3279, San Jose, January, 1998; EP 0 880 303; and “Ink-Jet Printing of Polymer Light-Emitting Devices”, Paul C. Duineveld, Margreet M. de Kok, Michael Buechel, Aad H. Sempel, Kees A. H. Mutsaers, Peter van de Weijer, Ivo G. J. Camps, Ton J. M. van den Biggelaar, Jan-Eric J. M. Rubingh and Eliav I. Haskal, Organic Light-Emitting Materials and Devices V, Zakya H. Kafafi, Editor, Proceedings of SPIE Vol. 4464 (2002). Ink jet techniques can be used to deposit materials for both small molecule and polymer LEDs.
A volatile solvent is generally employed to deposit a molecular electronic material, with 0.5% to 4% dissolved material. This can take anything between a few seconds and a few minutes to dry and results in a relatively thin film in comparison with the initial “ink” volume. Often multiple drops are deposited, preferably before drying begins, to provide sufficient thickness of dry material. Typical solvents which have been used include cyclohexylbenzene and alkylated benzenes, in particular toluene or xylene; others are described in WO 00/59267, WO 01/16251 and WO 02/18513; a solvent comprising a blend of these may also be employed. Precision ink jet printers such as machines from Litrex Corporation of California, USA are used; suitable print heads are available from Xaar of Cambridge, UK and Spectra, Inc. of NH, USA. Some particularly advantageous print strategies are described in the applicant's UK patent application number 0227778.8 filed on 28 Nov. 2002.
Inkjet printing has many advantages for the deposition of materials for molecular electronic devices but there are also some drawbacks associated with the technique. As previously mentioned the photoresist banks defining the wells have until now usually been tapered to form a shallow angle, typically around 15°, with the substrate. However it has been found that dissolved molecular electronic material deposited into a well with shallow edges dries to form a film with a relatively thin edge. FIGS. 4a and 4b illustrate this process.
FIG. 4a shows a simplified cross section 400 through a well 308 filled with dissolved material 402, and FIG. 4b shows the same well after the material has dried to form a solid film 404. In this example the bank angle is approximately 15° and the bank height is approximately 1.5 μm. As can be seen a well is generally filled until it is brimming over. The solution 402 has a contact angle θc with the plasma treated bank material of typically between 30° and 40° for example around 35°; this is the angle the surface of the dissolved material 402 makes with the (bank) material it contacts, for example angle 402a in FIG. 4a. As the solvent evaporates the solution becomes more concentrated and the surface of the solution moves down the tapering face of a bank towards the substrate; pinning of the drying edge can occur at a point between the initially landed wet edge and the foot of the bank (base of the well) on the substrate. The result, shown in FIG. 4b, is that the film of dry material 404 can be very thin, for example of the order of 10 nm or less, in a region 404a where it meets the face of a bank. In practice drying is complicated by other effects such as the coffee ring—effect. With this effect because the thickness of solution is less at the edge of a drop than in the centre, as the edge dries the concentration of dissolved material there increases. Because the edge tends to be pinned solution then flows from the centre of the drop towards the edge to reduce the concentration gradiant. This effect can result in dissolved material tending to be deposited in a ring rather than uniformly. The physics of the interactions of a drying solution with a surface are extremely complicated and a complete theory still awaits development.
Another drawback of banks with a long-shallow taper is that an inkjet droplet that does not fall exactly into a well but instead lands in part on the slope of the bank can dry in place, resulting in non-uniformities in the end display.
A further problem with inkjet deposition arises when filling wells which are large compared with the size of an inkjet droplet. A typical droplet from an inkjet print head has a diameter of approximately of 30 μm in flight and the droplet grows to approximately 100 μm in diameter when it lands and wets out. However it is difficult to produce drops of, say 100 μm in diameter (in flight) from a print head.
Filling a well or pixel of a similar size to a drop presents little problem as when the drop lands it spreads out and fills the well. This is illustrated in FIG. 5a which shows a well 500 for a long thin pixel of a type which is typically used in a RGB (red green blue) display. In the example of FIG. 5a the pixel has a width of 50 μm and a length of 150 μm with 20 μm wide banks (giving a 70 μm pixel pitch and a 210 μm full colour pitch). Such a well can be filled by three 50 μm droplets 502a, b, c as shown. Referring now to FIG. 5b this shows a well 510 for a pixel which is approximately four times larger than each dimension giving a pixel width of approximately 200 μm, more suitable for applications such as a colour television. As can be seen from the figure, many droplets 512 are needed to fill such a pixel. In practice, these tend to coalesce to form a larger droplet 514 which tends not to properly fill corners of the pixel (although FIGS. 5a and 5b and idealised and, in practice, the corners are not generally as sharp as they are shown). One way around this problem is to sufficiently over fill the well that the dissolved material well is pushed into the corners. This can be achieved by using a large number of dilute droplets and a high barrier around the well. Techniques for depositing large volumes of liquid are described in WO03/065474, which describes the use of very high barriers (for examples at page 8 lines 8 to 20) to allow the wells to hold a large volume of liquid without the liquid overflowing to adjacent wells. However such structures cannot easily be formed by photolithography and instead a plastic substrate is embossed or injection moulded. It is also desirable to be able to fill a well using fewer (higher concentration) droplets as this enables, inter alia faster printing.
One solution to the aforementioned problems is to modify the bank structure as described in the present applicant's earlier application GB-A-0402559.9.
Another problem associated with ink jet printing of organic opto-electrical devices such as those discussed above is that in the resultant device, the organic hole injecting layer can extend beyond the overlying organic semi-conductive layer providing a shorting path between the cathode and the anode at an edge of the well. This problem is exacerbated if the contact angle of the conductive organic composition with the bank material is too low. This problem is further exacerbated if the conductivity of the organic hole injecting layer is too high.
One solution to the aforementioned problem is to modify the bank structure by, for example, providing a stepped bank structure which increases the length of the shorting path, thus increasing the resistance of the path resulting in less shorting. Such a solution has been proposed by Seiko Epson. However, providing a more complex bank structure is expensive and increases the complexity of the manufacturing method for the device.
In relation to the aforementioned problem, the more conductive the organic composition, the greater the shorting problem will become. Thus, addition of polyol solvents to PEDOT in order to increase their conductivity as described in WO 2003/048229 (which discloses PEDOT with ethylene glycol, diethylene glycol and glycerol), WO 2003/048228 (which discloses PEDOT with diethylene glycol), and Polymer (2004), 45(25), 8443-8450 (which discloses PEDOT with ethylene glycol) will exacerbate this problem. Furthermore, although ink jet printing is mentioned in passing in these documents, the deposition techniques exemplified in these documents are not ink jet printing and the formulations disclosed appear to be too viscous for ink jet printing as a result of the high concentration of the polyol solvents utilized in these compositions.
The present applicant seeks to solve, or at least reduce, the problems outlined above by adapting compositions for ink jet printing comprising conductive or semi-conductive organic material. These adapted compositions are of particular use in the manufacture of light-emissive devices.
The feasibility of using ink jet printing to define hole conduction and electroluminescent layers in OLED display has been well demonstrated. The particular motivation for ink jet printing has been driven by the prospect of developing scalable and adaptable manufacturing processes, enabling large substrate sizes to be processed, without the requirement for expensive product specific tooling. In this application the implication of scalable and adaptable criteria for an ink jet print process are discussed and it is demonstrated how this can be achieved by development of suitable ink formulations.
The last five years have seen an increasing activity in the development of ink jet printing for depositing electronic materials. In particular there have been demonstrations of ink jet printing of both hole conduction (HC) and electroluminescent (EL) layers of OLED devices by more than a dozen display manufacturers. A number of these companies have set up pilot production facilities and have indicated that mass manufacture will start in 2007-2008 timeframe [M. Fleuster, M. Klein, P. v. Roosmalen, A. de Wit, H. Schwab. Mass Manufacturing of Full Colour Passive Matrix and Active Matrix PLED Displays. SID Proceedings 2004, 4.2].
The key reasons for the interest in ink jet printing are scalability and adaptability. The former allows arbitrarily large sized substrates to be patterned and the latter should mean that there are negligible tooling costs associated with changing from one product to another since the image of dots printed on a substrate is defined by software. At first sight this would be similar to printing a graphic image—commercial print equipment is available that allow printing of arbitrary images on billboard sized substrates. However the significant difference between graphics printers and display panels is the former use substrates that are porous or use inks that are UV curable resulting in very little effect of the drying environment on film formation. In comparison, the inks used in fabricating OLED displays are ink jet printed onto non-porous surfaces and the process of changing from a wet ink to dry film is dominated by the drying environment of the ink in the pixel. Since the printing process involves printing stripes (or swathes) of ink (corresponding to the ink jet head width) there is an inbuilt asymmetry in the drying environment. In addition OLED devices require the films to be uniform to nanometer tolerance. It follows that to achieve scalability and adaptability requires control of the film forming properties of the ink and a robustness of this process to changes in pixel dimensions and swathe timing. In this application it is demonstrated how this can be achieved with suitable ink engineering.
In general terms, the behaviour of drying drops of HC and EL inks is explained by the coffee-ring effect first modelled by Deegan [R. D. Deegan, O. Bakajin, T. F. Dupont, G. Huber, S. R. Nagel, and T. A. Witten. Capillary flow as the cause of ring stains from dried liquid drops. Nature 389, 827 (1997)]. For the case of circular pixels the wet ink forms a section of a sphere, where the angle made by the drop surface with the substrate is the contact angle. When pinning occurs (which it invariably does for the inks and surfaces used in polymer OLED display manufacturing) the drying drop maintains its diameter and solute is carried to the edges of the drop forming a ring of material at the outer edges of the pixel. The amount of material carried to the edge depends on a number of factors—in particular how long the process of material transfer can occur before the drying drop gels and the uniformity of the drying environment. At a swathe edge more drying occurs on the unprinted side since the solvent concentration in the atmosphere above the substrate is less than the printed side. With more evaporation taking place on the unprinted side more solute is deposited on this side and the film profile becomes asymmetric.
Embodiments of the present invention seek to solve the problem associated with a rapid change in the profile of organic layers within the pixels and between the pixels surrounding a swathe join.